Precision in the PARI library (gp_read_str)

[Rafael Guglielmetti <rafael.guglielmetti@unifr.ch>]

Dear PARI users,
I would like to compute the eigenvalues of a real symmetric matrix with 
PARI in a C++ program. To create the matrix, I use several information I 
read in a text file, create a string and use gp_read_str.

My problem: if one of the entry of the matrix is "sqrt(3)" for example, 
then the correspondent t_GEN has DEFAULTPREC and I'd like BIGDEFAULTPREC.
My question is the following: how can I increase the precision of 
gp_read_str?

Thanks.

Regards,

Rafael